Generally the laptop’s keys has this fine grain type of matt surface, the longer you use your laptop, it starts to wear away. Hold your laptop at a certain angle and you can find certain area on keys glossing over and shining an oily shine.
You can take a look at spacebar (at lest my spacebar) and easily tell whether I am left handed or right handed. So, what can be tone of the possible ways of hacking or “overlooking” it :-
1) If there is some major form filling season going on in the person’s life, just look for the shiniest keys and then sit in a corner trying to work out all the possible permutations or combinations
2) If the life is quite normal in the person’s life, assume his/her password is stored in the computer, look at the least shiny keys.
P.S. Of course, there are blaring pitfalls, but that is not the point, you see.